A coherent optical receiving system in an optical transmission system has become important with the request for increasing the capacity in the communication environment. In the coherent optical receiving system, information is transmitted by means of applying information to be transmitted to either or both of optical amplitude and optical phase. In order to receive the information applied to the amplitude or the phase, the received light is mixed with local oscillation light whose light frequency is nearly equal to that of the received light, and the interfering light arising from the mixture is detected by an optical detector and transformed into an electrical signal. The transformed electrical signal is digitally processed by a digital signal processing circuit in a following stage, and then a demodulated signal is obtained.
In such optical transmission system, a polarization multiplexing technology, in which two separate signals are multiplexed and transmitted using two orthogonal polarized waves (X and Y) of light, makes it possible to achieve twofold spectrum efficiency compared with that in an optical transmission system using a single polarized wave. In these days, the polarization multiplexing coherent optical receiving technology, therefore, has been used in many optical transmission systems.
FIG. 1 is a block diagram illustrating an example of a configuration of a common optical receiving device to which the polarization multiplexing coherent optical receiving system is applied.
An optical receiving device 500 receives, in the coherent receiving system, signal light in which a modulated carrier wave is polarization-multiplexed, as input signal light. As an example of a modulation system, there are a phase-shift keying modulation and a quadrature amplitude modulation. The phase-shift keying modulation is exemplified here as a modulation system.
Signal light propagating through an optical fiber transmission line (not shown) enters a polarization beam splitter 511 in the optical receiving device. The polarization beam splitter 511 splits the entered signal light into an X-component polarized wave and a Y-component polarized wave. Each split component is output to optical hybrid circuits 521 and 522 corresponding to each component. Local oscillation light output from a local oscillation light source 560 is also split into an X-component polarized wave and a Y-component polarized wave by a polarization beam splitter 512 and output to the optical hybrid circuits 521 and 522 corresponding to each component.
Each of the optical hybrid circuits 521 and 522 mixes the input signal light with the local oscillation light, and outputs a couple of light beams whose phase are different from each other by 90 degrees, that is, I (In-phase) component and Q (Quadrature) component.
The I component light and the Q component light are input into O/E (Optical/Electrical) conversion units 531 and 532 respectively, photoelectric-converted and output as analog electrical signals with the gains adjusted. The analog electrical signals are input into A/D (Analog/Digital) conversion units 541 and 542 respectively, sampled at appropriate time intervals and converted into quantized digital signals.
Thus the input optical signal is converted into one digital signal component (IX and QX: In-phase and Quadrature in X polarization) and the other digital signal component (IY and QY: In-phase and Quadrature in Y polarization), and digitally processed by a digital signal processing unit 550.
A polarization state of a signal in the optical fiber transmission line is made to change due to various external factors such as a pressure applied to the optical fiber. As a result, although the signal is multiplexed on the X polarization and the Y polarization respectively as separate signals at the transmitting side, the signal is received at the receiving side including a signal multiplexed in the other side of the X polarization and the Y polarization because a crosstalk arises during the optical fiber transmission. That is to say, the (IX, QX) component and the (IY, QY) component of digital signals before being input into the digital signal processing unit 550 are in an incomplete polarization demultiplexed state. The received signal also includes a transmission distortion due to transmission through the optical fiber.
The digital signal processing unit 550 performs an equalization processing of the digital signal in such state, demodulates the signal which has been modulated and transmitted at the transmission side and outputs the demodulated signal.
FIG. 2 is a block diagram illustrating an example of a configuration of a butterfly FIR (Finite Impulse Response) filter used in the digital signal processing unit 550.
The butterfly FIR filter performs the equalization processing for polarization mode dispersion and the polarization demultiplexing, and its function in outline is described below.
The butterfly FIR filter includes FIR filters and a coefficient control units 611 and 612 which generate tap coefficients to provide each FIR filter. Complex adders 631 and 632 are also included which add extracted signals having the same polarization component. In FIG. 2, each of the FIR filters is represented by an hxx filter 621, an hxy filter 622, an hyx filter 623, and an hyy filter 624.
For example, a signal multiplexed on the X polarized wave at the transmission side is represented by h, and a signal multiplexed on the Y polarized wave is represented by v. At the receiving side, as described above, the X polarized wave including the signal v and the Y polarized wave including the signal h are received.
In FIG. 2, a signal corresponding to an electric field of each polarization is input. An X polarization input signal EX is input into the hxx filter 621 and the hyx filter 623, and a Y polarization input signal EY is input into the hyy filter 624 and the hxy filter 622.
By adding the X polarization input signal optimally weighted by the hxx filter 621 to the Y polarization input signal optimally weighted by the hxy filter 622 in the complex adder 631, the signal v multiplexed on the X polarized wave and the signal v multiplexed on the Y polarized wave are canceled, and the signal h is output as the X polarization output signal Ex.
Similarly, by adding the Y polarization input signal optimally weighted by the hyy filter 624 to the X polarization input signal optimally weighted by the hyx filter 623 in the complex adder 631, the signal h multiplexed on the Y polarized wave and the signal h multiplexed on the X polarized wave are canceled, and the signal v is output as the Y polarization output signal Ey.
Thus, the butterfly FIR filter demultiplexes and outputs an original signal multiplexed on each polarized wave so as to output the signal h as the X polarization output signal Ex and the signal v as the Y polarization output signal Ey.
The coefficient control unit 611 and 612 monitor output signals from the complex adders 631 and 632, and always have control to provide adaptively optimum filter tap coefficients, so that the butterfly FIR filter can function as described above.
The polarization state of the signal in the optical fiber transmission line fluctuates fast due to various external factors. In order to control the above-mentioned polarization demultiplexing appropriately, therefore, it is necessary to match the polarization direction of the signal assumed at the receiving side with the polarization direction at the transmitting side in some way. As an algorithm to control tap coefficients for the polarization demultiplexing in such digital signal processing, a CMA (Constant Modulus Algorithm) is commonly known (see Non Patent Literature 1, item 5, for example).
The tap coefficient of the butterfly FIR filter is updated according to the updating rule exemplified in Non Patent Literature 1. The CMA is mainly used in order to demultiplex the signal in which an M-value phase modulation signal with constant amplitude of a signal, in particular, a QPSK (Quadrature Phase Shift Keying) signal, is multiplexed. The updating the tap coefficient using the CMA is performed so that the amplitude of an output signal may become constant. The updated coefficient of the butterfly FIR filter by that means converges so as to have characteristics reverse to the effect given to a transmission signal in the transmission line, and the polarization demultiplexing and the polarization mode dispersion compensation are carried out.
The Least Mean Square (LMS) algorithm is known as another algorithm used for performing the polarization demultiplexing. This is the algorithm including a decision of a received signal, and it is necessary to secure the convergence of the initial tap coefficient by using a training sequence. The use of such training leads to an increase in a line rate for a constant payload. On the other hand, the CMA is a blind signal processing without the decision of the received signal, and has an advantage in a convergent rate of tap coefficient and its simple configuration.
Patent literature 1 discloses an adaptive blind equalization device which is available for the polarization multiplexing coherent optical receiving device. The adaptive blind equalization device in patent literature 1 can prevent, without providing the training sequence, the problem in the CMA that the output signal of the X polarization component and the output signal of the Y polarization component converge to the same information source. The adaptive blind equalization device appropriately stops and restarts the output of the filter coefficient from a filter coefficient updating unit for the X polarization or one for the Y polarization, which operates independently, so that each output of the polarization components does not converge to the same information source.
Patent literature 2 discloses a filter coefficient adjusting device, in a polarization demultiplexer for an optical coherent receiver, which solves the problem in the CMA that two channels of an output signal converge to the same signal source. The filter coefficient adjusting device adjusts a filter coefficient so that the probability density of the output signal may approach a target of the probability density as much as possible. The target probability density is calculated under the condition that two channels of the signal converge to different sources.
Non Patent Literature 2 points out that, in the polarization demultiplexing system using the CMA, it is impossible to demultiplex appropriately the polarizations of the signal which is multiplexed in the polarization and modulated by the BPSK (Binary Phase Shift Keying). And Non Patent Literature 2 discloses a technology to solve the problem about the polarization demultiplexing for the BPSK signal by using CMA. The system disclosed in Non Patent Literature 2 utilizes a product of the output signals at two consecutive time points in either the X polarization component or the Y polarization component which is output from a butterfly FIR filter.    Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 2009-253972    Patent Literature 2: Japanese Patent Application Laid-Open Publication No. 2009-296596    Non Patent Literature 1: S. J. Savory, ‘Digital filters for coherent optical receivers,’ Opt. Express Vol. 16, No. 2, 804 (2008)    Non Patent Literature 2: Meng Yan et al., ‘Adaptive Blind Equalization for Coherent Optical BPSK System’ ECOC 2010, 19-23 Sep. 2010, Torino, Italy